This invention relates to ventilators for use in assisting patients to breathe, and, more particularly, to the triggering of such ventilators responsive to the breathing of the patient.
The condition of a patient who suffers from respiratory difficulties or other health problems can often be remarkably improved simply by ensuring a regular air supply that permits the energy of the patient to be directed elsewhere than obtaining sufficient oxygen. Many ill persons are therefore placed onto a program of breathing assistance with a device called a "ventilator". In simplest terms, the ventilator either forces pressurized gas into the lungs (e.g., a positive-pressure ventilator) or expands the chest cavity to draw gas into the lungs (e.g., a negative-pressure ventilator such as an iron lung) under a selectable schedule of gas composition, pressure, and flow pattern.
Although negative-pressure ventilators enjoyed a degree of popularity in the past, their use has been largely replaced by positive-pressure ventilators. The positive-pressure ventilator is a mechanical device external to the patient, which creates an external pressure and thereby forces gas into the patient's lungs through a tube termed the "airway". The gas may be air, pure oxygen, air enriched with additional oxygen, or some other oxygen-containing mixture.
Where the patient is attempting to breathe on his or her own, termed a "spontaneous breath", under some modes of breathing assistance the operation of the ventilator is synchronized to the spontaneous breathing of the patient so that the ventilator is not forcing gas into the lungs at the same time that the patient is attempting to breathe out. If the operation of the ventilator is not properly synchronized to the breathing of the patient, the ventilator actually works against the spontaneous breathing of the patient. In the absence of proper synchronization, the power of the ventilator can overcome breathing efforts of infants or weakened adults, and can do more harm than good.
To accomplish synchronization of the ventilator to the spontaneous breathing of the patient, a sensor is provided to sense the initiation of a spontaneous breath. The sensor output is used to generate a trigger signal for the ventilator, which then operates to reinforce the spontaneous breath. There is a lag time between the initiation of a spontaneous breath by the patient and the actual flow of gas from the ventilator to the airway, for several reasons. It is therefore important to provide a trigger signal that is as early in time, but after the patient initiates the spontaneous breath, as possible. That is, the sensor should sense the initiation of a spontaneous breath as quickly as possible after the breath is initiated.
Various sensor approaches have been used in the past. One is a pressure or flow sensor placed into the airway of the ventilator. Another type of sensor is a pneumatic sensor placed on the chest of the patient over the diaphragm, so as to sense the first movement of the diaphragm at the start of a spontaneous breath. Electrical sensing of diaphragm and muscle activity have also been used. Studies show that the trigger signal of the airway sensor lags the initiation of a spontaneous breath by a significant period of time. The trigger signals of the pneumatic or electrical diaphragm and muscle sensors lag the initiation of a spontaneous breath by a shorter, but still significant, period of time. Although these types of sensors can provide satisfactory results in some cases, there is an ongoing search for better types of sensors and breathing assistance methods to aid in synchronizing the ventilator to the spontaneous breathing of the patient.
There exists a need for a better approach to the synchronization of a ventilator to a patient's own breathing. Such an improved apparatus would significantly improve the potential for respiratory care of the patients. The present invention fulfills this need, and further provides related advantages.